It is known in the art to produce diffractive optical elements which manipulate light beams, an example light beam manipulation being the conversion of planar or spherical wavefronts to generalized wavefronts. The diffractive optical elements are generally thinner, lighter and more versatile than standard optical elements and are, therefore, desirable for use laser beam handling systems such as laser scanners, compact discs, laser computerized processing, laser radars and bar code scanners.
The article, "Micro Fresnel Lenses" by H. Nishihara and T. Suhara in Progress in Optics XXIV, edited by E. Wolf, presents background information on diffraction lenses of the Fresnel type.
The article, "Blazed Synthetic Phase-Only Holograms," by H. Dammann, published in Optik 31, 1970, pp. 95-104, discusses the efficiencies of diffractive lenses for use with holograms.
A method of manufacturing diffractive optical elements and its usefulness is discussed in the following articles by Gary J. Swanson and Wilfred B. Veldkamp:
"Diffractive Optical Elements for Use in Infrared Systems," Optical Engineering, June 1989, Vol. 28 No. 6, pp. 605-608;
"Infrared Applications of Diffractive Optical Elements,", SPIE Vol. 883: Holographic Optics: Design and Applications (1988), pp. 155-162;
"Developments in Fabrication of Binary Optical Elements," SPIE Vol. 437: International Conference on Computer-generated Holography, 1983, pp. 54-59.
The optical elements produced by the method of Swanson and Veldkamp have a multiplicity of phase zones whose shape is defined by the optical element phase profile and is approximated by a series of steps. Each zone has the same number of steps and the width of each step of a zone varies according to the phase profile of the zone. The number of steps per phase zone defines the efficiency of the optical element, where, for example, eight steps produce a optical element with roughly 95% efficiency and sixteen steps produce one with 99% efficiency. Unfortunately, the smallest zone dictates the number of steps which limits the efficiency of the optical element.
The method includes the step of generating N masks which, when used to etch the material of an optical element, produce 2.sup.N steps in each phase zone. Every step has the same height but the width varies according to the zone shape.
The efficiency of the optical elements produced increases discretely with the number of masks used since each mask doubles the number of steps. The method does not provide a way to produce an optical element with an efficiency between that of the discrete levels.
The abovementioned efficiency measurements are for a wide beam which impinges upon the optical element. For a narrow beam scanning along the optical element, as is common in laser scanners, the efficiency of the optical element produced via the abovementioned method will vary across the optical element. This is due to the fact that the width of the steps vary. If a step is as wide or wider than the beam width, the narrow beam will effectively impinge on a flat surface.
U.S. Pat. No. 4,846,552 describes a method to produce a binary planar optical element utilizing holographic and very large scale integration (VLSI) techniques.